The present invention relates to a rotating electrical machine, of which a rotor comprises a cage conductor, and a drive system of a cage induction motor.
JP-A-7-231630 discloses a rotating electrical machine, in which rotor conductors of a cage induction motor are sectioned along a plane perpendicular to a shaft to comprise a lower portion enlarged stepwise in width as compared with an upper portion positioned near to an outer periphery of a rotor. Also, “Transformer/induction machine/AC commutator motor” published by Institute of Electrical Engineers, 6th edition, Nov. 21, 1983, pages 311 to 315 (FIG. 3.63, FIG. 3.64, and FIG. 3.67) discloses a rotating electrical machine, in which rotor conductors have the same cross sectional shape as that in JP-A-7-231630 and the cross sectional shape of the rotor conductors over a whole height thereof tapers linearly toward an outer periphery of a rotor to be trapezoidal in shape.
The rotor conductors disclosed in JP-A-7-231630 have an inverted T-shaped cross section such that the upper portions of the rotor conductors positioned near to the outer periphery of the rotor are small in width and the lower portions are enlarged stepwise in width. Therefore, high resistance is generated at the starting of a rotating electrical machine, so that starting characteristics of the rotating electrical machine can be improved. On the other hand, since the rotor conductor is enlarged stepwise in width and the rotating electrical machine is sharply reduced in secondary resistance, the rotating electrical machine is reduced in torque. Also, coming-out of the rotor conductors due to centrifugal forces is restricted only by the steps of the rotor conductors. Therefore, centrifugal forces concentrate on the steps and the rotor conductors are in some cases broken by friction. Further, since the rotor conductors are sharply changed in width, a high technique is needed for processing of the rotor conductors in the case where a drawn material is used for the rotor conductors.
Meanwhile, with the rotating electrical machine of “Transformer/induction machine/AC commutator motor”, the cross sectional shape of the rotor conductors tapers over a total height thereof toward the outer periphery of the rotor to be trapezoidal in shape, so that high resistance is generated at the starting of the rotating electrical machine and a starting characteristics of the rotating electrical machine is improved. Since the rotor conductors are trapezoidal-shaped, however, a height thereof and a width of a bottom thereof are determined by restriction in strength, there is little freedom in design, and a starting characteristics and a steady-state characteristics of the rotating electrical machine are not balanced in some cases.
It is an object of the invention to provide a rotating electrical machine having starting characteristics of low starting current and high starting torque, and a good balance between a starting characteristics and a steady-state characteristics.
It is a further object of the invention to provide a drive system of a cage induction motor, in which alternating current with a commercial frequency remaining intact is supplied directly to the induction motor to realize a characteristics of low starting current and high starting torque, and a characteristics of high efficiency and high power factor.
Other objects and features of the invention will be made apparent from the following description of the embodiments.